Does your tire says "MAX PSI 80 psi." or something similar?

 My answer is that while those words or similar may be on the sidewall, those words are NOT to be taken literally as an instruction because those words are in reality only PART of an important phrase.

In reality your tires more likely say something like "Max Load 2500 pounds at 80 PSI Max Inflation". Of course different vehicles will have different size and type tires but we are looking at some general guidelines here.

Do you see and understand that the actual information the tire company is telling you is that the tire has a maximum load capacity of 2,500 pounds when it is inflated to 80 Psi? Most people understand that the load capacity of a tire is controlled and limited by the inflation in the tire, so this phrase is telling us that the load capacity of the subject tire will never be greater than 2,500 pounds even if you increase the inflation pressure

The loads and inflation are published in industry standards books such as the US Tire & Rim Association. Almost all tires made for sale and use in the US will follow the industry standards book AKA "TRA".

 I believe that the RV company engineers either have copies of the books or at least copies of the applicable pages.

 As always, you need to remember we are ALWAYS talking about the "cold" inflation which means when the tire is at Ambient air temperature and has not been driven or in direct sunlight for the previous couple of hours. If I ever talk about the warm or hot inflation that will be made obvious in that post.

Every vehicle has a Certification Label. This image is from a Class-A RV.

The specifics of this individual label are not important as you should have captured a nice, sharp picture of the label for your RV.  Class-A has the label usually near the driver's left elbow, Class B & C and LT have the label in the driver door jam and towables (TT and 5th wheel trailers) have the label on the outside, driver side, toward the front of the trailer.

The label states the Tire Size, and the recommended inflation if it is a passenger car. If it is an RV it will give the GAWR, Tire Size, Tire Load Range, and the inflation required to support the GAWR. Passenger car and pick-up truck inflation is arrived at after years of testing and evaluation of different tire designs from different manufacturers at different inflation that balance the requirements on a long list (hundreds of items) from the vehicle company. These requirements include Force & Moment test results and fuel economy plus dozens of detailed ratings within each category for Ride, Handling, and Noise. Car inflation is a "Recommendation" from the engineers at the car company aimed at giving you the performance they designed into the car.

I have never heard of RV companies evaluating tires. It seems that they generally look for the lowest-cost tire that can provide the load capacity required by law. This usually means for towables that the tires would need to be inflated to the level required to deliver the max load capacity. A few RV companies are selecting better tires with Nylon Cap Ply but you need to pay attention.

The inflation number on a tire sidewall is the PSI needed to deliver the Max Load rating stated on the tire. Normally tire load capacity is increased with an increase in inflation and that would normally mean you would increase the inflation, BUT since the tire was selected for its max load capacity as stated on the tire sidewall, the inflation that will deliver the highest load capacity for that tire is the "MAX infl" which is NOT the highest inflation the tire can tolerate.

Yes, the wording can be confusing but the wording was not selected with the average user in mind.
Motorhomes are built on chassis built and designed by vehicle manufacturers so the tires on Motorhomes will many times have inflation on the label that is lower than the tire max because the tires were evaluated and selected by the vehicle chassis manufacturer.

Many towables are built with minimal capacity for additional "stuff" but many people load their RV with no thought to the load capacity. This is why the data shows that over half the RV on the road have one or more tire or axle in overload. This is a major contributor to tire failures and why there are so many RV trailer tire failures.

Too often on RV forums, people do not pay attention to what type of vehicle is being discussed A Passenger car (or 1/2 tom Pickup) or a Motorhome or a Trailer (5th wheel). These three completely different types of vehicles require three different guidelines and inflation information. This failure to be specific with the vehicle type is why people get confused.

 

What does "MAX Inflation" on tire sidewall mean?

 "MAX PSI" is a hot topic on various RV forums I monitor, so let's jump in. I have answered this question previously but some folks have decided they know more than the tire companies.

First, as always, you need to remember we are talking about the "cold" inflation which means when the tire is at Ambient air temperature and has not been driven or in direct sunlight for the previous two hours. If I ever talk about the warm or hot inflation that will be made obvious in that post.

Every vehicle has a Certification Label.

This one happens to be from the side of a Class-A  RV. The specifics of this individual label are not important as you should have captured a nice, sharp picture of the label for your RV.  Class-A has the label usually near the driver's left elbow, Class B & C and LT have the label in the driver door jam and towables (TT and 5th wheel trailers) have the label on the outside, driver side, toward the front.

The label states the Tire Size, and the recommended inflation if it is a passenger car. If it is an RV it will give the GAWR, Tire Size, Tire Load Range, and the inflation required to support the GAWR. Passenger car and LT inflation is arrived at after years of testing and evaluation of different tire designs from different manufacturers at different inflation that balance the requirements on a long list (hundreds of items) from the car company. These requirements include Force & Moment test results and fuel economy plus dozens of detailed ratings within each category for Ride, Handling, and Noise. Car inflation is a "Recommendation" from the engineers at the car company aimed at giving you the performance they designed into the car.

I have never heard of RV companies evaluating tires. They generally look for the lowest-cost tire that can provide the load capacity required by law. This usually means for towables that the tires would need to be inflated to the level required to deliver the max load capacity. A few RV companies are selecting better tires with Nylon Cap Ply but you need to pay attention.
The inflation number on a tire sidewall is the PSI needed to deliver the Max Load rating stated on the tire. Normally tire load capacity is increased with an increase in inflation and that would normally mean you would increase the inflation, BUT since the tire was selected for its max load capacity as stated on the tire sidewall, the inflation that will deliver the highest load capacity for that tire is the "MAX infl" which is NOT the highest inflation the tire can tolerate.

Yes, the wording is confusing but the wording was, not selected with the average user in mind.
Motorhomes are built on chassis built and designed by vehicle manufacturers so the tires on Motorhomes will many times have inflation on the label that is lower than the tire max because the tires were evaluated and selected by the vehicle chassis manufacturer.

Many towables are built with minimal capacity for additional "stuff" but many people load their RV with no thought to the load capacity. This is why the data shows that over half the RV on the road have one or more tire or axle in overload. This is a major contributor to tire failures and why there are so many RV trailer tire failures.

Too often on RV forums, people do not pay attention to what type of vehicle is being discussed A Passenger car (or 1/2 tom Pickup) or a Motorhome or a Trailer (5th wheel). These three completely different types of vehicles require three different guidelines and inflation information. This failure to be specific with the vehicle type is why people get confused.

Roger Marble

Check out my Blog www.RVTireSafety.Net

Have a tire question? Ask Roger on his RV Tires Forum here. It's hosted by RVtravel.com and moderated by Roger. He'll be happy to help you.

Read more from Roger Marble on RVtravel.com.

 ##RVT1110

What does "MAX PSI" on the sidewall mean?

 

I know I have answered this question in the past but maybe it was on an RV forum for a single-brand RV so not everyone has heard this. So here goes.

Many, but not all, tires have the words "Max PSI" followed by a number on the tire sidewall. I have no idea which lawyer wrote the requirement but he or she didn't understand how those words would be misinterpreted.

Too often I see people saying that the tire should never be run with a higher pressure or that this is the absolute only correct pressure for the tire, but these assumptions are incorrect.

I hope everyone reading this post understands that tire load capacity is related directly to tire inflation and if they want to increase the load capacity they will need to increase the inflation. While it is true that an increase in tire inflation is required if you want more load capacity, there is a limit. The limit is controlled by industry standards which are published and followed by all tire companies.

Each tire has a Maximum Load Capacity and to get to that capacity you need to increase the tire inflation BUT there is a limit as each tire also has a limit or maximum load capacity and increasing the inflation above the stated pressure WILL NOT increase the load capacity.

So you have a tire that says "Max Psi 65 psi" which means you will gain increased load capacity as you increase inflation from 35 to 45 to 60 and to 65 psi BUT any additional pressure above 65 psi WILL NOT RESULT in any additional increase in load capacity.

It is also important to know that tires are tested and can tolerate higher pressures due to being warmed up by running so the pressure on the tire sidewall only refers to the tire "cold" inflation so you should not bleed down the hot pressure.

 

Have a tire question? Ask Roger on his RV Tires Forum here. It's hosted by RVtravel.com and moderated by Roger. He'll be happy to help you.

Read more from Roger Marble on his blog at RVtiresafety.net or on RVtravel.com.

 ##RVT1098

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Should you use the inflation that is listed on the tire sidewall?

 Many RV owners do not understand the inflation and load information that is molded on the sidewall of the tires they own.  As a tire design engineer, part of my responsibility was to specify all the words, symbols and numbers on the tire sidewalls. Some of the words such as brand name like as Goodyear or Michelin and the "line"  like  "Eagle" or "Transforce" are obviously selected by the sales and marketing departments. 

The tire size or sizes had been selected and it was my job to design and request molds appropriate for the sizes that were to be part of the new line of tires. For each size, I needed to consult the US Tire & Rim Association data book and to specify the appropriate load and inflation numbers for the maximum allowable for the Load Range of each tire. With LT lines we might have LR-C, LR-D and LR-E tires and if incorrect numbers ended up on the tires they would need to be recalled and replaced for free so it was obvious that this information had to be correct.

With the exception of a few tires that are designed for a new model of a specific car or Light Truck from Ford, GM, Toyota, M-B etc, I would never know what vehicle the tires would end up being mounted on so there would be no way for me to specify a specific load or inflation for any given tire that was vehicle specific. Selecting the tire and Load Range and specifying the inflation number to be posted on the vehicle Certification label is, by regulation, the responsibility of the vehicle manufacturer. It was however my responsibility to ensure the tire was capable of passing the DOT tests as well as company minimums based on the use of industry standard loads and inflation numbers.

All tires have a Maximum Load capacity molded on the tire sidewall along with the minimum inflation needed to support that load.

With this background information I hope you now understand that the inflation number molded on the tire sidewall is not selected or made up by the tire engineer as the inflation number comes from the published Industry standards. The inflation on the Certification Label aka Tire Placard is specified by the RV manufacturer and by federal Regulation must be sufficient to support the GAWR (Gross Axle Weight Rating) which is part of the RV company design process.

The placard inflation may be equal or less than the number on the tire sidewall but it should never be higher than the tire sidewall pressure number.

Ideally you would have actual scale loads with the RV fully loaded at its expected heaviest. With that load known you can consult the tables to learn the minimum inflation needed to support your load. However if you don't have scale readings the best thing to do is inflate your tires to the placard information. This is OK as long as you have not overloaded your tires. Note that the tables for tires made by Goodyear are the same as for tires made by General, or Firestone, or Bridgestone, or Cooper. Also most Michelin tires have the same numbers as the other tire companies with the exception of a few tires that were originally designed for European country application where metric units are standard. When converting to the US "Inch-Pound" units the numbers do not always match up so if you have Michelin tires you probably need to consult their tables.  You can also check the tables listed on this blog post.

 

If you review the above you will see that I am not advising that you use the inflation on the tire sidewall, unless that is the inflation on your Placard.

A side comment on the inflation number of the tire. The words may be "Max Inflation" on some tires, but this is related to the way loads are established for tires based on the inflation. The LIMIT is the stated maximum load. The intent in the wording is to stress that the tire load can not be increased even if the inflation is increased above the number associated with the tire's "Maximum Load".

Regarding hot tire inflation numbers higher than the number on the tire sidewall, that is OK as long as the set "cold inflation" is not set higher than the number on the tire. Tire inflation will increase when the tire temperature increases. The rate  of change is about 2% for a change of 10F. Seeing your "Hot" inflation higher than the tire sidewall is not a big deal. With the advent of TPMS many people are seeing tire temperature or pressure increase for the first time as a driver. Most TPMS have the high temperature set to 158F.  I have found that setting the high pressure to 125% of your cold set pressure should normally avoid having the alarm sound as long you are not overloading your tire or pushing your driving speed. If you get close you might just drop the speed back a few mph. Personally I set my cruse to 62 or 63 mph and stay in the right lane. I do not get people honking and I get real good fuel economy. (10.2 mpg in my last 1,200 mile trip from Ohio to WY).

 

##RVT1008

Max pressure

A clarification might be of help. The pressure number molded on the sidewall of tires is NOT the "Maximum Allowable" pressure. It is the Cold pressure necessary to support the Maximum Load capacity for that tire. In reality the pressure number molded on the tire sidewall is the Minimum needed to provide for the support of that load.

Increasing the cold pressure above the number on the tire sidewall will NOT increase the tire rated load capacity per industry practice, standard and guidelines.

If the pressure increases because of increased Ambient Temperature or because the tire gets hot from being in the Sunlight or the tire gets hot from being driven on, that increase is considered and accounted for by tire design engineers. The increase is about 2% for each increase of 10°F in tire temperature.  Even a temperature increase of 100F ( 20°F to 120°F for example)would only result in about 20% increase in tire pressure and undamaged tires can tolerate a greater pressure increase than 20%.

Note: I am not saying that you can heat a tire to above 190F and not have problems but those problems would be the result of high temperature degradation of the rubber and not simply due to pressure increase.

 

##RVT974

I bleed off the high air pressure so I don't exceed the "Max Pressure"

The above statement or a variation of it seems to be made about once a week on one or another of the various RV forums I frequently review. It appears that many people incorrectly believe that exceeding the 'Max Pressure" number molded on the tire sidewall is going to result in tire failure and explosion.
It may help to let people know about some of the testings that tires undergo during the development phase.

First some background. The "Max Inflation" statement on your tire sidewall really is the inflation associated with the Maximum Load capacity. If you actually read your tire sidewall you will see that is what it says. So the number of PSI is really the minimum needed to support the stated load. It is important to understand that we are always talking about the "cold" inflation and not the inflation of a tire that is running down the highway or has been in direct sunlight or driven on in the prior couple of hours. "Cold" inflation does not mean the tire needs to be refrigerated but it means the tire is at the prevailing Ambient temperature.

With the introduction of aftermarket TPMS that report both tire pressure and the temperature of the sensor, people are now being exposed to numbers, they have no experience with. They see tire pressure rising as they drive down the highway. Some may see a 10% rise in pressure from the pressure they set their tires to just a half-hour earlier. Others may see a 20% or even a 25% increase in pressure and for some, this increase was a cause for concern. Not because they had any working knowledge of tire operating temperature or pressure but simply because the pressure was higher than they expected. A few people have decided that they need to bleed down the high pressure because they thought the pressure number of the tire sidewall was the max it should ever see.  Of course, the action of bleeding down the hot pressure was exactly the wrong thing to do as that meant that the tire would no longer be operating at the pressures expected by the tire engineer when they originally designed and tested the tire specification.

Obviously, this raises the question of how much pressure a tire can tolerate before the owner should be concerned. Well, I am going to give you some numbers but these are only examples. I cannot speak for every tire companies process of specifications but if we start with a few guidelines I think you can get comfortable and hopefully, you will believe that tire design engineers do have a reasonable idea of what they are doing.

First off we are only going to discuss regular production street tires that have passed DOT testing. The numbers I will use would basically be seen on new tires. I cannot advise on a tire's capability after it has been damaged or run for many thousands of miles. Damaged tires can fail when re-inflated as seen in THIS video. Heavy truck multi-piece rims MUST be inflated in a safety cage. If you suffered a puncture and drove any distance with the tire underinflated you may have permanently damaged the body cords which is why you should always tell the tire service person you drove on the under-inflated tire so they know to use a cage or other restraining safety equipment. Age and miles can reduce the strength of any tire but we do consider this degradation when approving a design. Another consideration is the availability of high-pressure air. Most home compressors or those that supply air to hoses made available to consumers have an upper limit of about 150 psi so we don't expect any consumer to have access to or to use industrial air pressure which can be 400 psi or even special inflation units as used on aircraft tires.

Basically many tires are tested to 300 to 400% of their rated inflation or above 200 psi whichever is lower. I had a number of tires that could contain over 300 psi when new but if they had been damaged the same tire might fail at less than 200psi but I also never, in my 40 years saw a tire fail from normal pressure increase and I was involved in the testing and evaluation of thousands of tires.

The BOTTOM LINE is Please do not bleed down your hot inflation pressure. You should ONLY set and adjust inflation when the tire is cool and at Ambient temperature. Doing otherwise MAY result in you having a failure days or weeks later because you were running lower pressure than what was needed for the tire load you were applying.

Max Load - Max Inflation & Cold Inflation confusion continues

From an RV Forum "My understanding is that the number on the side of the tire is not a recommendation. Rather it is the maximum cold temperature that the tire is rated for. You should never run above that number. Without actual weights, I would run the recommendation from the motorhome manufacturer on the placard near the driver's seat, or a little higher, but never over the sidewall number."

The information on the tire sidewall is a statement of the maximum load capacity and the cold inflation required to provide that load capacity.

Different tires have the word "max" placed in different parts of the phrase which leads to the confusion.

Here is a post on passenger tires.

Here is a post on when the Minimum inflation is the Maximum.

Note: A a statement of Max Load "At" stated inflation such as "Max load 3,000 lbs at 80 psi" is leaving off the implied "minimum" before the 80 psi.

Also introducing the word "cold" seems to confuse some.

Maybe this will help:

Tire inflation should only be set when the tire is at ambient temperature and has not been warmed by being in sunlight or driven on for the prior two hours.

A tire may be loaded to the number of pounds stated on the sidewall, only when the tire has been inflated, when cold, to the stated pressure on the tire sidewall.

The problem is that these phrases don't fit in the same place as current "Max Load 3,000 lbs at Cold inflation 80 psi"

You can learn some facts about Max tire pressure by reading your tire

I found significant discussion on tire 'maximum pressure" on a forum thread on a new Michelin tire. One problem is that the new tire is not yet available in the market so getting confirmation on what the tire has for markings on the sidewall was based on second hand information from various Michelin sources. Despite these problems I felt it might be instructive to review the relationship between load capacity and inflation pressure.

The following was posted:

The Michelin LTX M/S2 P235/75R15XL 108T carries its rated load at 41 PSI. Yet the maximum pressure stated on the tire is 50 PSI. There is no load capacity increase above 41 PSI.
Tire Rack has a good explanation of the maximum load for a tire
Tire Tech Information - Tire Specs Explained: Maximum Load

As I have previously reported the numbers associated with the "complete" size nomenclature are very important. In the above thread there were a number of readers confusing the inflation associated with the max load capacity with a statement on the tire about safety and tire inflation. When discussing Load and inflation it should be easy to read the tire as the max load and the inflation needed to support that load are clearly stated together.

There is also a "Safety Warning" on many passenger and similar info on LT tires that includes a statement about "max inflation" but that number is really the max pressure that should ever be used when inflating a tire to ensure proper seating of the tire bead against the rim. This number is not associated with the load carrying capacity of the tire.

Here is a sample Safety Warning from a Performance 16" passenger tire



Here is the Load and inflation information, again for a passenger tire



Now while the above are from a passenger tire that was convenient for me to get pictures of, I know that you should see similar on your ST or LT type tires.

I think you may learn a bit more about your own tires if you take a moment to read the information provided. Knowing what tires say on the sidewall can help avoid making mistakes when assumptions are made.

 It might be a good idea to take a few minutes to go and read your tire sidewall. You might be surprised what you can learn.


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